Underwater pipelines can be continuously laid from a surface vessel employing S-Lay, Natural J-Lay or Forced Vertical J-Lay mechanical arrangements. Each of these methods have the pipeline being laid approaching the ocean floor in a catenary curve.
S-Lay systems have the pipeline bent back from its near catenary curve to an almost horizontal position where strings of pipes can be added on a vessel deck. Natural J-Lay systems (called J-Lay systems in short) keep the pipeline in its natural near catenary attitude. New pipes have to be brought up at a slanting angle to match the angle of the upper end of the pipeline in the water. Forced Vertical J-Lay systems have the top end of the pipeline bent further from the near catenary curve so as to bring it to a vertical position where new pipes can to be added in a vertical tower.
Both the first and the third type use so-called xe2x80x9cstingersxe2x80x9d to bend the pipeline to the desired attitude for welding new pipe sections. The second type requires a pipe clamping device sometimes also called improperly a xe2x80x9cstingerxe2x80x9d.
S-Lay arrangements offer the definite advantage of a near horizontal pipeline on vessel deck, allowing in-line multiple welding, testing and coating stations but require long and, in deep water, deep, expensive and relatively fragile stingers to bend the pipe from its natural no moment angle in the water to the near horizontal on the vessel deck.
Forced Vertical J-Lay systems allow the use of fixed vertical pipe laying towers but also require a sometimes very deep stinger. In addition, keeping the stingers within reasonable dimensions sometimes induce plastic deformation of the pipe, or permanent plastic deformation. In large diameter pipelines, the moment required to handle the upper end of the pipeline can be substantial.
Natural J-Lay systems do not require genuine stingers, strictly speaking, but at the cost of a neither horizontal nor vertical laying attitude, thus involving complex articulated towers. Current natural J-Lay arrangements demand the provision of complex upending or erecting strongback arms to bring new pipes or strings of pipes to a non horizontal variable position where they are jointed to the existing deployed pipeline.
The three kinds of arrangements require that the pipeline total weight be supported above deck in clamps or friction tensioners, the weight of the pipe being held back from the bottom or the top of the systems. Whether J-Laying or S-Laying, that provision is a real drawback when the job calls for the installation of large manifolds inline, as the size of the manifold is bound to be limited by the dimensions of the tensioning or weight holding device. In addition, near vertical J-Lay arrangements where the weight of the deployed pipeline is supported from the top of the tower require very strong structures, thus limiting the overall capacity of the system.
Natural J-Lay Systems have historically been designed as modified onshore drilling rigs. Little of the specific marine environment taken into consideration and all operations are carried on above vessel deck level until the pipeline is eventually lowered into the water. Those systems use drawworks, ram-rig type cylinders or near vertical friction pipe tensioners to hold back the weight of the deployed pipeline, strongback pipe erectors to upend new strings of pipe and rotating articulated masts to allow for a variable pipe angle at water level. In addition, some designs integrate mechanical gimballing of the whole system to compensate for weathervaning vessel rotation.
The object of this invention is to provide a system for laying pipeline from a vessel with a tower at a fixed angle, but allowing the lower end of the new pipe sections to be aligned with the suspended pipeline by flexing the new pipe sections.
A second object of the present invention is to suspend the pipeline with a multiplicity of winches.
A third object of the present invention is to allow weathervaning of the vessel around the suspended pipeline.
Another object of the invention is to suspend the load of the pipeline below the deck of the vessel rather than above the deck of the vessel.
Another object of the invention is to allow for handling of relatively large subsea packages in the work area while handling the load of the pipeline below the working table area.
Another object of the invention is to provide an area to feed relatively short pipe sections into the tower for welding together in the tower.
Another object of the present invention is to provide the ability to lay pipelines at a variety of angles from a fixed angle tower, without requiring the inducement of a moment on the top of the pipeline.
Another object of the invention is to do the required pipe bending on the portion of the pipeline which is not under tension.